A model for the ergosterol biosynthetic pathway in Chlamydomonas reinhardtii
Ergosterol and 7-dehydroporiferasterol are the predominant sterols in the membranes of the alga Chlamydomonas reinhardtii Dangeard. Ergosterol is primarily found in most fungi, which produce their end-product sterols by way of the intermediate lanosterol. In contrast, plants rarely make ergosterol and plant sterols are made via the intermediate cycloartenol. The cycloartenol-lanosterol bifurcation has been used as a means to evolutionarily categorize species based on their sterol production. Use of bioinformatics has revealed that the green alga, C. reinhardtii is probably producing sterols using a pathway very similar to that of higher plants. The Chlamydomonas genome was searched for genes that encoded proteins exhibiting high similarity to sterol biosynthetic proteins in the higher plant Arabidopsis thaliana or the yeast Saccharomyces cerevisiae. Genes with the greatest similarity were chosen and annotated. To establish whether these genes were expressed, the presence of their transcripts was determined and quantitative RT-PCR was performed. It was observed that many of the transcripts coordinately increased in abundance after deflagellation, which induces an increase in membrane biosynthesis. This work demonstrates that C. reinhardtii has all of the genes necessary for the biosynthesis of ergosterol and 7-dehydroporiferasterol and that this alga uses the higher plant pathway to make a sterol normally associated with fungi. Evidence is also presented that other organisms commonly known as algae, including chlorophytes, haptophytes, rhodophytes, bacillariophytes, a phaeophyte and a cryptomonad, are likely to utilize the same biosynthetic pathway, making sterols via a cycloartenol intermediate.